Race Fuel

You will only have to rejet after going to race fuel.

VP C12 or Phillips B33 will tend to make the bike run richer, and will require leaner jetting.

Oxygenated fuels like VP MR1 or MR2, or Phillips B35 will tend to make the bike run leaner, so will require richer jetting.


Just curious...If the Yamalube was working well, why did you switch? Because it was cost prohibitive? Perhaps the Klotz syn oil had friction modifiers, which as stated with *any* oil that contains these, can cause clutch slippage problems. I'd go back to the Yamalube4R. I've used it and not had any problems.


I spoke to Klotz and was told that Yamalube 4R was not a good choice. Imagine that. Cost is not an issue. The Klotz was the same if not more..bought a case?


Are you running race fuel? If so, which one...what was the benefit? Cost is not an issue....if it helps the bike run better...prevents detonation.

I have seen a few posts about people using Race Fuel. I understood that the timing on the 01 426 is setup for 92 unleaded? I would prefer to run race fuel due to it being more consistent. Currently I am just running Amoco 92. Stock jetting. What's the word? What are some of you running? I Ran straight C12 in my YZ250. If you run race fuel will I need to change timing settings? or just jetting?

Also, raced up at Muddy Creek this weekend,...Tennessee Kawasaki State Championships....James Stewart ripped them alive in the 125pro....every class! Sorry, back to the question...I was getting a lot of false neutrals this weekend. First time I have had this happen. I am bad about shifting up/down without the clutch. I ran Klotz synthetic oil this weekend. I was running Yamalube 4R with no problems. Any ideas?

Thanks for the help

I think VP C14 is the best fuel for the 426. I run it straight in my WR and have not seen a need to significantly change the jetting. I think good racing fuel in sealed containers is just more consistent than pump gas - makes jetting easier. My $.02 - keep the change.

The 426 has a redline of 11,200 RPM. As an engine approaches the higher RPM. . . which is considered to be above 8,000 RPM by today's automotive standard, then there is less time to compress, fire, and make power from this air/fuel mixture. Average pump gas just isn't distilled to make use of this RPM range. This is where race fuel comes into play. 92 octane pump gas is...well...not very consistent. The octane level might be adequate, but the distillation curve won't cut it for our machines. It just won't fire quick enough. As RPM's rise, there is less and less time to fire the mixture. Race fuel fills this void. Don't expect to pour high octane fuel into your tank and think that your 426 or 259F is going to instantly run like Arnold on steroids. It doesn't work like that. It could...and probably...WILL run worse. The 426 will respond to the correct fuel(throttle response wise) if you are willing to jet crisp. In my opinion, the thing that makes the 426 so much fun is to have it respond exactly the way you expect it when you twist the trottle. If it hiccups, spits, spurts, hesitates, or otherwise is annoying, it's just not fun. Believe me, it's possible to have instantaneous throttle response. Ferry has it. My '00 has it and it's notorious for the hiccup.

[This message has been edited by Boit (edited 07-11-2001).]

I am not running race fuel at this point because of availability. I would run Phillips B35 if I could get it regularly. I have heard very good things about it and the cost is very good. If you can find it you should be able to get it for about $3-5 per gallon. But whatever is available to you for a reasonable cost would certainly be better than pump fuel if for nothing else, consistency.

VP MR1, which is what Team Yamaha uses in the YZ250F and 426F is very high priced, somewhere between $15-20 per gallon. I would expect it is somewhat comparable to Phillips B35 although I haven't compared them specifically. MR2 is about the same price I think.

The above mentioned fuels are oxygenated. VP C12 works very well also, as does Phillips B33, which are not oxygenated.

The benefit to running race fuel for the weekend warrior would for most part be in the consistency of the fuel and thus the jetting. Once you get the bike jetted for the fuel then you should realize some better throttle response and maybe even some slight power gains. I haven't heard of many people needing race fuel to prevent detonation as these bikes tend to run fine on pump premium.

I am a new conversion from a kx to 01yz 426.

I love the new bike. I have a garage full of various high compression internals combusted toys. I keep a barrel of b-32 on hand.

the new bik came with a full tank of who knows what? It ran great. with b-32 it is not as crisp as it was. an at low speed

like just putsing in the yard it cut's out

doesn't die but like spontainously flicking the kill button. Think it can be tuned with the needle position and air screw? the manual shows all kinds of needles slide pilots and mains. I am sure i can can get it tuned but souunds like a lot of good knolage out there. Oh yah I'm in minnesota at 900ft

Sounds like C12 or C14 is the way to go....I have a 15 gallon drum of C12..ran it in my YZ250..so I think I will jet it for that fuel. Anyone running C12 in the Southeast (GA) and if so what is the best jetting specs you have found? .... engine is completely stock ... maybe ten hours on her. Thanks for the help.

MGR8: Expect to go at least one step leaner on the main jet...possibly two steps at this time of year. I found the stock pilot to be in the proper range with just a slight leaner setting on the fuel screw. Each bike seems different so your setting may not mirror mine. I'm just outside of New Orleans so our altitude, temperatures, and humidity are pretty similar.


I am about to install a DSP pipe/silencer and I am going to jet to run VP C12. Can you give me your jetting specs so I can use them as a base line? Thanks

I'd stay away from that Klotz stuff. Before I got my 426 I put that Klotz synthetic in my '98 YZ250, and my buddy changed his CR125 at the same time and used the Klotz too (both of us on the recommendation of the local Yamaha dealer). We both went from almost never having false neutrals, to having them on almost every lap.

After a day at the track, we went back to the local Yamaha dealer and he admitted that he had advised a number of people to switch, and just about all of them had come back in to complain about missed shifts, etc.



'01 YZ426

'00 Harley Police Special(Got it for free, and I get paid to ride it!)

I'm running a #158 main jet and everything else is stock except for the bottom accelerator plate. I have to adjust the fuel screw almost continuously to get the off-idle response that I want. I installed the Factory R&D bottom plate to try and rid the bike the bike of that annoying hesitation when the throttle is whacked open suddenly. The leaner main jet is needed to offset the rich jetting caused by the C-12. The #158 is what I've found to work great now that it is so damn hot. Once the ambient temperature drops to below about 55 degrees F, I start going richer. A #160 works well in this range...especially at night. If it gets down into the 30's or low 40's, I use the stock #162. It's pretty easy to tell when the main jet is a little too rich because the engine wants to blubber at high RPM. I just bought 5 gallons of VP MR2 to experiment with. Rich Rohrich has given some excellent advice and guidance on what this fuel should do. It's oxygenated and leaded so I will probably need to jet richer right away. The rewards should be an even better throttle response than with C-12 although I can't imagine anything better than what I have now...but, it will be fun and good for my further education with jetting. BTW, Sunoco has just come out with a fuel that seems ideal for the 426 and 250F machines. It's called MX114 and has the distillation curve that we need for our high revving engines. My fuel supplier doesn't have it yet and didn't even know about it when I asked him last Tuesday.

[This message has been edited by Boit (edited 07-28-2001).]

[This message has been edited by Boit (edited 07-28-2001).]

Hey, just a little tid bit about race fuel guys. I wrench on GSXR race bikes and we use Nutec race fuel in our bikes on race day. One thing to keep in mind, if you keep race fuel in your bike and in your carb all of the time, it will eventually deteriorate the seals in your carb. It's potent stuff, at least Nutec is. We run pump gas for practice, siphon it out, put in Nutec and then race. Once it's over, we pump out the race fuel using a siphon and put pump gas back in. We then run the engine for a couple of minutes to get pump fuel back into the carb and through the motor. I was reading a back issue of DB and they mentioned the same thing. Just thought I'd let any of you know that run Hi Test year round. Besides, running Hi Test fuel could be extremely costly for year round use. :)


Thanks for the jetting specs. Let me/us know how the MR2 works. The guy who does my suspension work (he also does killer motor work) said that MR2 does wonders for 4 strokes. Looking forward to hearing how it works.

Are you running a aftermarket pipe/silencer?...what brand?

Just incase your not aware....the 100 octane avation fuel (blue,100LL)is leaded. The LL stands for low lead. I dont have my owners manual handy, but I dont think leaded fuel is recommended for the 400-426. Be careful, I worked as an aircraft mechanic for a few years and have seen a number of street car racers develop engine problems due to continous use of 100LL

Just something to think about. I read a while back back in a motocross magazine (the name escapes me)that you should not run anything but pump gas in the YZ400-426. The race gas notoriously eats at the valve seals and causes premature wear. I suppose you could justify needing race gas and risk it if you are constantly feeling as though the 426 just doesn't hit hard enough. I suppose that's why the pro's do it...and the fact that money and maintenance aren't an issue. I'm only repeating what I read.

MGR8: I went with the White Brothers E series "S" bend titanium hi-boy head pipe and carbon fiber silencer. With this setup, along with proper jetting and good fuel, the engine is nothing short of phenomenal. . . especially right off idle and into the mid-range. It signs off a little earlier than the stock pipe but since I don't like to hit the rev limiter, it's fine with me. The tracks where I live don't have high speed straights, so having a good strong bottom end is highly desirable.

coloradothumpin: The M/X magazines have printed some bogus information in the past and the "race fuel vs carb seals" is one of them. M/X Action is notorious for publishing misinformation.

[This message has been edited by Boit (edited 08-02-2001).]

Just my cent-I have been using 100 aviation fuel in my bike for quite a while now and it seems to work great with the stock jetting. It runs a lot better than pump gas but it still only costs about $1.40 a gallon and doesn't have the same effect on the carbuerater seals as some of the race gases.


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